The turbocharger for an internal combustion engine for use, for example, in automobiles and ships, is constructed by providing a compressor wheel (compressor impeller) 2 whose rotating axis is identical with that of a turbine wheel (turbine impeller) 1 rotated by exhaust energy, as illustrated in FIG. 1. The compressor wheel 2 is provided for feeding air compressed by high-speed rotation to an internal combustion engine 3. In FIG. 1, reference numeral 4 denotes air, reference numeral 5 denotes compressed air, and reference numerals 6 and 7 denote flow of an exhaust gas at respective sites. Reference numeral 8 denotes a shaft connecting the turbine wheel 1 to the compressor wheel 2. FIG. 2 shows an example of the shape of the compressor wheel. The compressor wheel is configured so that a plurality of thin blades 11 protrude out from a disk 10 integrated with a rotation center shaft (boss) 9. The compressor wheel is heated at a temperature as high as about 150° C. during high-speed rotation, while the vicinity of the center of rotation, particularly the disk, suffers high stress caused by torsional stress and centrifugal force from the rotation shaft.
The compressor wheel is constructed with various materials depending on the demand for performance of the turbocharger. The wheel is generally shaped by cutting an aluminum alloy hot-forged material, for use in larger-size engines, such as for ships. However, for relatively smaller wheels for automobiles engines, such as for passenger cars and trucks, or small-size ships engines, easily-castable aluminum alloys containing Si as a major additive element, for example, those good in castability as defined in JIS-AC4CH (an alloy of Al-7% Si-0.3% Mg), ASTM-354.0 (an alloy of Al-9% Si-1.8% Cu-0.5% Mg) and ASTM-C355.0 (an alloy of Al-5% Si-1.3% Cu-0.5% Mg), are cast in a plaster mold, by a low-pressure or reduced-pressure casting method or a gravity casting method, and the cast alloy is subjected to a solution treatment and/or an aging treatment, to strengthen to be widely used, since mass productivity and production cost are emphasized. The basic production methods thereof are disclosed in detail in U.S. Pat. No. 4,556,528.
Meanwhile, a high compression ratio of air has been required in the turbocharger in recent years, to improve output power of the internal combustion engine, and high-speed rotation is naturally required for this purpose. However, heat values generated by air compression by increasing the rotation speed are increased, and the turbine wheel at the exhaust side is heated at a high temperature. Consequently, the compressor wheel is also heated at a high temperature, due to heat conduction from the turbine wheel. It has been revealed that continuous normal rotation is impossible in a compressor wheel made of the above conventional easily-castable aluminum alloy containing Si as a principal additive element, since it is apt to cause such trouble as deformation during operation and further breakage by fatigue. In particular, while the upper limit available for use in the existing conventional compressor wheel is about 150° C., development of a compressor wheel capable of use at a temperature of about 180° C. has been strongly required to attain the objects described above.
Accordingly, it may be conceived to change the composition of the aluminum alloy to another composition excellent in high-temperature mechanical strength, for example, an alloy defined in JIS-AC1B (an alloy of Al-5% Cu-0.3% Mg). However, since the compressor wheel has a complex shape with thin blade portions thereon, fluidity of a molten alloy of this alloy is so poor that the molten alloy tends to cause miss run in the thin portions (poor filling), as described in paragraph [0011], page 2, in the specification of JP-A-10-58119 (“JP-A” means unexamined published Japanese patent application). Accordingly, JP-A-10-58119 proposes a method in which an easily-castable alloy, such as an Al—Si-series alloy, for example, AC4HC, is used for the blade portion that emphasizes run of the molten alloy, while a high-strength alloy, such as Al—Cu-series alloy, for example, AC1B, is used from the boss portion to the disk portion, where sufficient strength is required to join the rotation shaft; and, the molten alloys of these alloys are independently poured into the mold in two steps, followed by combining the two portions, to form the compressor wheel. JP-A-10-212967 proposes a method for forming the compressor wheel in which an alloy good in castability is used for the blade portion, while a composite reinforced material, prepared by strengthening a reinforce material, such as 25% B-aluminum whiskers, which is impregnated with aluminum, is used at the portion from the boss portion through the central portion of the disk that suffers from stress; and these portions are separately produced, and are joined thereafter to form the compressor wheel. JP-A-11-343858 proposes to join these portions by friction welding.
As described above, no compressor wheels made of cast aluminum alloys that are durable to an increased temperature caused by an increase of the rotation speed have been industrially manufactured using a single alloy. Further, the methods described above, in which different materials are independently used for the blade portion and boss portion, respectively, have not been industrially applied yet, since these methods are poor in productivity to result in increase of the production cost.